Methods of making sheathed electric heating units



Apnl 4, 1967 E. F. DILLON 3,311,969

METHODS OF MAKING SHEATHED ELECTRIC HEATING UNITS Original Filed Nov. 1, 1961 2 Sheets-Sheet 1 INVENTOR. EUGENE E. DILLON BY I 0% M64472 I may) I 07% ATTORNEYS April 4, 1967 E. F. DILLON METHODS OF MAKING SHEATHED ELECTRIC HEATING UNITS Original Filed Nov. 1, 1961 *9 F 6 5 F/6.6 A

V 0:0 3 0 0 00 a 00 a O 04 2 Sheets-Sheet 2 EUGENE F. DILLON ATTORNEYS United States Patent ()fiice 3,311,969 Patented Apr. 4,1967

3,311,969 METHODS OF MAKING SIEATIED ELECTRIC HEATING UNITS Eugene F. Dillon, Oak Park, 11]., assignor to General Eiectric Company, a corporation of New York Original application Nov. 1, 1961, Ser. No. 149,381, now

Patent No. 3,195,093, dated July 13, 1965. Divided and this application Nov. 23, 1964, Ser. No. 413,228

20 Claims. (Cl. 29-15569) The present invention relates to methods of making sheathed electric heating units of thehermetically sealed type. This application is a division of the copending application of Eugene F. Dillon, Ser. No. 149,381, filed Nov. 1,1961, now Patent No. 3,195,093.

A conventional sheathed electric heating unit of the hermetically sealed type comprises an elongated resist ance conductor embedded in a mass of highly compacted electrical-insulating and heat-conducting material, such as granular magnesium oxide, and arranged in an elongated enclosing metal sheath, as Well as a pair of metal terminals disposed at the opposite ends of the sheath and electrically connected to the opposite ends of the resistance conductor. The terminals project from the opposite ends of the sheath, and the inner ends thereof are embedded in the compacted granular material, whereby both the resistance conductor and the pair of terminals are securely retained in place and electrically insulated from the sheath. The opposite ends of the sheath are closed by a pair of glass seals respectively surrounding the pair of terminals; and a charge of air is hermetically sealed within the sheath in permeating relation with the compacted granular material.

It has now been discovered that when a conventional electric heating unit of this type is placed in service, the charge of air that is trapped or hermetically sealed in the sheath is gradually depleted, so that ultimately a substantial vacuum is formed in the sheath. Specifically, the oxygen in the air is depleted by oxidizing of the interior of the sheath, and the nitrogen in the air is depleted by nitrifying of the interior of the sheath. Also, there is some oxidizing and nitrifying of the resistance conductor. When such substantial vacuum is thus formed in the sheath, the thermal conductivity of the unit between the resistance conductor and the sheath through the compacted granular material is drastically reduced, with the result that the temperature of the resistance conductor is greatly increased; whereby the resistance conductor is subject to vaporization and ultimately fails by open-circuit after a relatively short time interval, so that the heating unit has a life expectancy that is shorter than is desirable.

It has also been discovered that the objectionable vacuum described above can be prevented by providing the sheath with an initial gas-filling that includes a substantial amount of an inert gas selected from the class consisting of helium and argon, whereby the inert gas is not depleted during the operation of the heating unit, so that the thermal conductivity through the compacted granular material is maintained at a relatively high value during prolonged operation of the heating unit, with the result that the temperature difference between the resistance conductor and the sheath is minimized, whereby the heating unit has a long useful life.

Accordingly, it is a general object of the present invention to provide a method of making a sheathed electric heating unit of the hermetically sealed type, wherein the method is essentially productive of such a heating unit having a long useful life.

Another object of the invention is to provide a method of the character noted, wherein the heating unit thus produced comprises a charge of inert gas contained in the sheath thereof, and wherein the objectionable depletion of the inert gas within the sheath is prevented during the operation of the heating unit.

A further object of the invention is to provide an improved method of making a sheathed electric heating unit of the hermetically sealed type that involves a simple procedure in the production of the hermetic seals at the opposite ends of the sheath thereof.

A still further object of the invention is to provide an improved method of making a sheathed electric heating unit of the hermetically sealed type that involves a simple procedure in the charging of the inert gas into the sheath prior to the production of the hermetic seals at the opposite ends of the sheath.

The present invention is predicated upon the aforesaid discovery that a conventional sheathed electric heating unit of the hermetically sealed type has undesirable short life expectancy by virtue of the depletion of the charge of air contained in the sheath thereof and the re sulting formation of a substantial vacuum in the sheath during the operation of the heating unit, coupled with the further disco-very that the undesirable substantial vacuum mentioned may be prevented by the initial charging of the sheath with an inert gas selected from the class consisting of helium and argon.

Further features of the invention pertain to the particular steps of the method of making the sheathed electric heating unit mentioned, whereby the above outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the ac comnanying drawings, in which:

FIGURE 1 is an enlarged fragmentary longitudinal sectional view, partly broken away, of the upper end of a sheathed electric heating unit in an initial stage of manufacture thereof, and made in accordance with the present method;

FIG. 2 is an enlarged fragmentary longitudinal sectional view, partly broken away, of the lower end of the heating unit shown in FIG. 1;

FIG. 3 is an enlarged fragmentary longitudinal sectional view, partly broken away, of the upper end of the heating unit in a subsequent stage of manufacture thereof;

FIG. 4 is an enlarged fragmentary longitudinal sectional view, partly broken away, of the lower end of the heating unit shown in FIG. 3.

FIG. 5 is a further enlarged fragmentary longitudinal exploded sectional View, partly broken away, of the upper end of the heating unit in a still subsequent stage of manufacture thereof; and

FIG. 6 is a still further enlarged fragmentary longitudinal sectional view, partlyv broken away, of the upper end of the heating unit after the final step of manufacture thereof.

Referring now to the drawings, there is illustrated a sheathed electric heating unit 10 that is made in accordance with the present method; which heating unit 10 comprises an elongated tubular metal sheath 11, an elongated helical resistance conductor 12 located substantially centrally within the sheath 11, and a highly compacted mass of heat-conducting and electrical-insulating material 13 arranged in filling relation with respect to the sheath 11 and in embedding relation with respect to the resistance conductor 12. Also the heating unit 10 comprises an upper metal terminal 14 and a lower metal termin-a1 15, each of substantially rod-like form. The upper terminal 14 is arranged at the upper end of the sheath 11 and projects from the exterior thereinto and in spaced 1 relation therewith, whereby the inner end of the upper terminal 14 is also embedded in the adjacent highly compacted mass of material 13. The lower terminal 15 is arranged in the lower end of the sheath 11 and projects from the exterior thereinto and in spaced relation therewith, whereby the inner end of the lower terminal 15 is also embedded in the adjacent highly compacted mass of material 13. The extreme inner end of the upper terminal 14 is shouldered to provide an inwardly projecting pin 16 of reduced cross-sectional area that is rigidly sistance conductor 12, as by welding. Likewise, the extreme inner end of the lower terminal 15 is shouldered to provide an inwardly projecting pin 17 of reduced crosssectional area that is rigidly secured within the adjacent lower end of the resistance conductor 12, as by welding. Specifically, the opposite ends of the resistance conductor 12 may be respectively secured to the pin-like structures 16 and 17 of the terminals 14 and 15 in accordance with the welding method disclosed in US. Patent No. 2,546,351, granted on Mar. 27, 1951, to Sterling A. Oakley.

In a construction example of the heating unit 10, the sheath 11 may be formed of Incoloy, a nickel-chromium steel comprising by weight about 30% nickel and about 20% chromium and the balance principally iron. Alternatively, the sheath 11 may be formed of 3098 stainless steel, 21 chromium-nickel steel comprising by weight about 25% chromium and about 12% nickel and the balance principally iron. The resistance conductor 12 may be formed of a suitable nickel-chromium alloy comprising by weight about 80% nickel and about 20% chromium. Each of the terminals 14 and 15 may be formed of a suitable low carbon steel, such, for example, as 1008 cold rolled steel. The highly compacted mass of material 13 may consist essentially of granular magnesium oxide.

Referring now to FIGS. 3 and 4, an upper plug 18 of ceramic material is arranged in the upper end of the sheath 11 in surrounding relation with respect to the intermediate portion of the upper terminal 14 and respectively bonded thereto, and a lower plug 19 of ceramic material is arranged in the lower end of the .sheath 11 in surrounding relation with respect to the intermediate portion of the lower terminal 15 and respectively bonded thereto. Each of the plugs 18 and 19 is 'of substatnially homogeneous, cellular and porous construction, so as to allow breathing of gas therethrough, for a purpose more fully explained hereinafter; and each of the plugs 18 and 19 may consist essentially of kaolin and silica and metal silicates and may be of the composition as disclosed in US. Patent No. 2,962,684, granted on Nov. 29, 1960, to Gunder Lien, J r. More specifically, this ceramic material has the approximate composition as follows:

Percent Kaolin 28 Silica 27 Zirconium silicate 36 Calcium carbonate 3 Borax Sodium silicate 1 This ceramic material in finely divided form is characterized by sintering into a unitary vitreous mass upon heating thereof to a predetermined elevated temperature in the approximate range 1900 F. to 2100 F.; and the material of the composition set forth has a softening point at approximately 2050 F. This material is further characterized by oongealing of the sintered mass upon cooling thereof below the temperature range mentioned into a substantially homogeneous, cellular and porous body of ceramic material.

Further, the heating unit comprises a pair of hermetic seals respectively arranged at the opposite ends secured within the adjacent upper end of the re.

of the sheath 11; and referring to FIGS. 5 and 6, the seal 20 there illustrated, that is arranged at the upper end of the sheath 11, essentially comprises a metal ferrule 21, a metal tube 22, and a body of gas-impervious and electrical-insulating material 23 arranged within the outer end of the ferrule 21 and surrounding the inner end of the tube 22 and providing an hermetic seal therebetween. As best shown in FIG. 6, the inner end of the ferrule 21 is disposed in closely surrounding relation with the upper end of the sheath 11 and is hermetically sealed thereto. More particularly, the extreme inner end of the ferrule 21 is shouldered to provide a tubular projection 24 thereon; and the extreme inner end of the tubular projection 24 is welded to the adjacent annular portion of the sheath 11, as indicated at 25, thereby to produce the previously mentioned hermetic seal between the upper end of the sheath 11 and the ferrule 21.

The inner end of the tube 22 is disposed interiorly of the outer end of the ferrule 21 and spaced therefrom, and the outer end of the tube 22 is disposed exteriorly of the outer end of the ferrule 21, and the body of gasimpervious and electrical-insulating material 23 is arranged within the outer end of the ferrule 21 and in surrounding relation with the inner end of the tube 22 and providing the previously mentioned hermetic seal therebetween. Preferably, the body of material 23 comprises a substantially annular mass of fused glass that is arranged in compression between the outer end of the ferrule 21 and the inner end of the tube 22 and intimately bonded thereto. The metal tube 22 is'also arranged in closely surrounding relation with the outer end of the upper terminal 14; and the extreme outer end of the tube 22 is hermetically sealed to the adjacent annular portion of the upper terminal 14, as by welding, as indicated at 26.

Accordingly, in the upper seal 20, the ferrule 21 is hermetically sealed by the annular welding bead 25 to the adjacent annular portion of the upper end of the sheath 11, and the tube 22 is hermetically sealed by the annular welding bead 26 to the adjacent annular portion of the outer end of the upper terminal 14, and the outer end of the ferrule 21 is hermetically sealed to the inner end of the tube 22 by the interposed annular body of fused glass 23. Also, in the seal 20, the extreme outer end of the upper plug 18 is disposed adjacent to the extreme inner end of the tube 22, and preferably in abutting relation therewith. Furthermore, the body of fused glass 23 that is arranged in the outer end of the ferrule 21 is positioned somewhat outwardly with respect to the extreme upper end of the sheath 11 to define a small chamber 27 therebetween that constitutes a gas reservoir, within the ferrule 21 that is employed for the purpose of holding a small quantity of the inert gases that are used to fill the sheath 11, as explained subsequently.

In a constructional example of the seal 20, the ferrule 21 is formed of a suitable low carbon steel, such, for example, as 1015 cold rolled steel that is free of sulphur, lead and phosphorus. The tube 22 is formed of a suitable nickel-iron alloy comprising by weight about 52% nickel and the balance principally iron. Also, the composition of the body of glass 23 is suitably selected so that it has a thermal coefficient of expansion that falls between that of the ferrule 21 and that of the tube 22, the thermal coefficient of expansion of the tube 22 being somewhat lower than that of the ferrule 21; which arrangement insures that the body of glass 23 is maintained under compression between the ferrule 21 and the tube 22, so that the seal 20 is characterized by high thermal and mechanical shock resistance.

Finally, it is noted that the heating unit 10 comprises a charge of inert gas hermetically sealed within the sheath 11 and permeating the compacted mass of granular material 13; which inert gas is selected from the class consisting of helium and argon. Preferably, the inert gas comprises by weight about 96% to 99% argon and about 1% to 4% helium, the helium inclusion facilitating ready testing for leakage of the finished heating unit 10, as explained more fully hereinafter.

Considering now the method of manufacture of the heating unit 10, in accordance with the present invention, and referring particularly to FIGS. 1 and 2, the preformed terminals 14 and 15 are welded to the adjacent opposite ends of the preformed helical resistance conductor 12; and a preformed bushing 19x formed of the ceramic material previously described is placed over the outer end of the lower terminal 15 and retained in place by a combustible washer 31 fitted over the lower terminal 15. The location of the ceramic bushing 19x and the Washer 31 is insured by a ring-like depression 11a that is formed in the lower end of the sheath 11 and by a groove 15a that is formed in the intermediate portion of the lower terminal 15. The preformed tubular sheath 11 is then placed in surrounding relation with the subassembly described, the subassembly being threaded through the sheath 11, so that the ceramic bushing 19x is located in the lower end of the sheath 11 in engagement with the associated annular shoulder 11a, and with the annular washer 31 arranged in the extreme lower end of the sheath 11, as indicated in FIG. 2. At this time, the upper terminal 14 projects from the upper end of the sheath 11; and in passing, it is noted that the upper terminal 14 comprises a knob-like part 14a disposed at the extreme upper end thereof, as shown in FIG. 1, that is adapted to cooperate with a loading machine, as explained more fully below. This combustible washer 31 is preferably formed of a suitable synthetic organic resin, such, for example, as polyethylene or polystyrene.

At this time, the assembly described is transferred to a suitable loading machine, such, for example, as that disclosed in US. Patent No. 2,316,659, granted on Apr. 13, 1943, to John L. Andrews. In the loading machine, the assembly is retained in a substantially vertical position with the enlarged knob-like part 14:: carried by the upper end of the upper terminal 14 cooperating with the hook incorporated in the loading machine and arranged substantially centrally with respect to the upper end of the sheath 11 and projecting outwardly therefrom. At this time, the loading machine is operated so that the magnesia in finely divided or granular form is charged into the upper end of the sheath flowing th-erethrough onto the ceramic bushing 19x and first embedding the inner end of the lower terminal 15. As the loading machine is operated, the sheath 11 is filled so that the resistance conductor 12 is embedded in the granular material 13 and ultimately the inner end of the upper terminal 14 is embedded in this granular material. During charging of the insulating material 13 into the sheath 11, the sheath 11 may be vibrated or jarred slightly in order to insure tamping or packing of the finely divided material 13 in the space between the terminals 15 and 14 and the sheath 11 and between the convolutions of the helical resistance conductor 12 and the sheath 11 and into the core of the helical resistance conductor 12. After the insulating material has been charged into the sheath 11 and tamped in place, filling the spaces mentioned within the sheath 11, the assembly is removed from the loading machine. At this time, a hollow void is defined in the upper end of the sheath 11; which hollow void is filled by placing a preformed hollow bushing 18x (identical to the bushing 19x) thereinto. More particularly, the bushing 18x is placed over the outer end of the upper terminal 14 in engagement with the upper end of the mass of insulating material 13 that has been charged into the sheath 11; and at this time, an upper combustible washer 32 (identical to the washer 31) is placed over the extreme upper end of the upper terminal 14 and retained in place by frictional engagement therewith and brought into contact with the upper end of the bushing 18x, within the extreme upper end of the sheath 11. In the completed assembly thus produced, as shown in FIGS. 1 and 2, the bushing 19x serves as a stopper at the lower end of the sheath 11, while the adjacent washer 31 serves to prevent the loss of the ceramic material of the bushing 19x from the adjacent lower end of the sheath 11 in the succeeding step of the method of manufacture. Similarly, the bushing 18x serves as a stopper at the upper end of the sheath 11, while the adjacent washer 32 serves to prevent the loss of the ceramic material of the bushing 18x from the adjacent upper end of the sheath 11 in the succeeding step of the method of manufacture.

The completed assembly of FIGS. 1 and 2 is then transferred to a rolling machine, such, for example, as that disclosed in US. Patent No. 2,677,172, granted on May 4, 1954, to Sterling A. Oakley; wherein the assembly is subjected to cold working in a plurality of successive cold rolling passes so as substantially to reduce the cross-sectional area of the sheath 11 for the purpose of compacting the finely divided magnesia so as to produce the highly compacted mass 13 in the finished heating unit 10. More particularly, the rolling machine may comprise a number of substantially elliptical rolling stages, arranged in angularly rotated relation between a first cylindrical rolling stage and a last cylindrical rolling stage, so that the sheath is materially reduced in the rolling machine, for the purpose explained. As an example, the sheath 11 may be initially cylindrical having an outside diameter of approximately 0.263", and the sheath 11 in the finished heating unit 10 may be substantially cylindrical and smooth having an outside diameter of approximately 0.238". Thus in the rolling machine, the outside diameter of the sheath 11 is reduced from 0.263' t-od 0.238" in the several cold rolling passes effecting a corresponding reduction in the cross-sectional area thereof, the reduction in the initial cross-sectional area of the sheath 11 being about 10% in the present example.

In passing, it is noted that in the operation of the rolling machine disclosed in the Oakley patent mentioned, the assembly is moved vertically through the successive rolling passes effecting the progressive reduction in the cross-sectional area of the sheath 11 as described.

From the rolling machine, the assembly is transferred to an electric annealing furnace provided with a reducing atmosphere, and arranged in a substantially horizontal position therein; whereby the temperature of the assembly is quickly raised from the ambient temperature to a predetermined elevated temperature in the temperature range 1900 F. to 2100" F. More particularly, in the present example, the temperature of the assembly is raised from the ambient temperature to an elevated temperature of about 2050 F. in about 8 minutes in the electric furnace; and this elevated temperature of the assembly is held for a short time interval of about 8 minutes; whereupon the assembly is removed from the electric furnace back to the air. As noted, the electric furnace contains a suitable reducing atmosphere, which, in the present example, has the approximate composition as follows:

Percent N 65 H 18 CO 11 CO 5 CH 1 In the operation of the rolling machine, the bushings 18x and 19x ofier no resistance to the rolls and are crushed and reduced to finely divided form; however, the respectively associated washers 32 and 31 prevent any substantial escape of the finely divided ceramic material from the respectively associated upper and lower ends of the sheath 11. When the assembly is transferred to the electric annealing furnace, the washers 31 and 32 are quickly destroyed by combustion leaving no carbon deposits upon the respective terminals and 14, and also the two masses of crushed ceramic material of the two bushings 19x and 18x are sintered into two respective unitary vitreous masses disposed in the respective opposite end of the sheath 11. More particularly, the mass of vitreous material in one end of the sheath 11 expands outwardly producing the meniscus in the plug 18 of the finished heating unit 10; and similarly, the mass of vitreous material in the other end of the sheath 11 expands outwardly producing the meniscus in the plug 19 of the finished heating unit 10. Also, the two masses of vitreous material flow into wetting and bonding relation with the adjacent interior portions of the sheath 11 and with respect to the adjacent portions of the respective terminals 14 and 15, without flowing from the opposite ends of the sheath 11 sufficiently to produce voids therein. Thereafter, following removal of the assembly from the electric annealing furnace, the two masses of vitreous material congeal into the two respective self-supporting cellular and porous ceramic plugs 18 and 19 intimately bonded to the respective ends of the sheath 11 and intomately bonded to the respective terminal-s 14 and 15, as previously explained.

Of course, it will be understood that the heat-treatment described not only brings about the formation of the cellular and porous ceramic plugs 18 and 19 in the opposite ends of the sheath 11, as shown in FIGS. 3 and 4 but also effects annealing of the sheath 11 for the purpose of relieving stresses induced therein in the cold working thereof in the preceding rolling operation.

In the manufacturing method, the assembly, as shown in FIGS. 3 and 4, comprises a first assembly and each of the seals 20, as shown in the upper portion of FIG. 5, comprises a second assembly. Of course, the first assembly is produced in the manner described above, while each of the second assemblies rsp roduced in any conventional manner.

In the manufacturing method, a group of the first assemblies are, in fact, heat-treated simultaneously in the annealing furnace described; and after these first assemblies are removed from the annealing furnace, they are placed as a group in an enclosure, while they are still at an elevated temperature. The enclosure is then closedup or sealed and a substantial vacuum is drawn therein, whereby the reduced pressure in the enclosure effects the extraction of air from the sheath 11 of each of the first assemblies through the associated porous plugs 18 and 19 sealed in the opposite ends thereof. More particularly, the interior of the enclosure may be subjected to a relatively low pressure, below about 20 mm. of Hg; which relatively low pressure is held for a time interval of about to 45 minutes, so as to allow a high degree of extraction of air from the sheath 11 of each of the first assemblies described. During this holding time interval, there is substantial cooling of the first assemblies contained in the enclosure; and at the expiration of the time interval mentioned, a charge of inert gas at a gauge pressure of about l0# per square inch is forced into the enclosure; which pressure of the inert gas is held in the enclosure throughout a time interval of about 30 minutes. As previously mentioned, the charge of inert gas that is thus held in the enclosure essentially comprises argon that contains a small amount of helium that is employed for a subsequent test purpose, as more fully explained hereinafter. Accordingly, the inert gas that is held under pressure within the enclosure penetrates the porous plugs 18 and 19 provided in the opposite ends of the sheath 11 of each of the first assemblies, thereby effecting charging of the sheath 11 with the inert gas, the inert gas permeating the highly compacted granular material 13 contained in the sheath 11 and embedding the resistance conductor 12 of the heating unit 10. At the conclusion of the second mentioned holding period, wherein the inert gas is maintained under pressure within the enclosure, the group of first assemblies are removed from the enclosure.

At this time, each of the first assemblies is worked into the finished heating unit 10, as shown in FIG. 6, by the incorporation thereinto of two of the second assemblies, as shown in FIG. 5. More particularly, one of the second assemblies is placed upon the upper end of the sheath 11 of the first assembly so that the inner end of the ferrule 21 is disposed in closely surrounding relation with the upper end of the sheath 11 and so that the outer end of the ferrule 21 is disposed outwardly of the upper end of the sheath 11 and so that the tube 22 is arranged in closely surrounding relation with the outer end of the upper terminal 14. At this time, the extreme inner end of the ferrule 21 is Welded to the adjacent annular portion of the sheath 11 to produce the annular welding bead 25 and consequently the hermetic seal between the ferrule 21 and the sheath 11. Also, the extreme outer end of the tube 22 is welded to the adjacent annular portion of the upper terminal 14 to produce the annular welding bead 26 and consequently the hermetic seal between the tube 22 and the upper terminal 14.

In a similar manner, another of the second assemblies is placed upon the lower end of the sheath 11 of the first assembly so that the inner end of the ferrule 21 is disposed in closely surrounding relation with the lower end of the sheath 11 and so that the outer end of the ferrule 21 is disposed outwardly of the lower end of the sheath 11 and so that the tube 22 is arranged in closely surrounding relation with the outer end of the lower terminal 15. At this time, the extreme inner end of the ferrule 21 is welded to the adjacent annular portion of the sheath 11 to produce the associated annular welding bead and consequently the hermetic seal between the ferrule 21 and the sheath 11; and the extreme outer end of the tube 22 is welded to the adjacent annular portion of the lower terminal 15 to produce the adjacent annular welding bead and consequently the hermetic seal between the tube 22 and the lower terminal 15.

These welding steps may be readily carried out in a convenient manner utilizing a heliarc welding machine employing argon as the inert atmosphere and a feed wire formed essentially of Incoweld No. 82 or 92 that essentially comprises a nickel-chromium alloy.

At this time, the heating unit 10 is in finished condition, the opposite ends of the sheath 11 being hermetically sealed by the two corresponding seals 20, each of the construction described above.

Reverting to the manufacturing method, when the first assemblies are removed from the enclosure after being subjected to the pressure of inert gas of about 10# per square inch gauge in the enclosure, it will be appreciated that the total charge of gas trapped in the sheath 11 of each of the first assemblies comprises both some air as well as the mixture of inert gases mentioned; whereby the total gas pressure within the sheath 11 of each of the first assemblies is somewhat above atmospheric pressure. Moreover, after removal of the first assemblies from the enclosure, the gas trapped in the sheath 11 of each of the first assemblies gradually leaks therefrom by virtue of the porous character of the ceramic plugs 18 and 19 sealed in the opposite ends of the sheath 11. However, this leakage of gas from the sheath 11 is at a very slow rate, whereby there is ample time for the placement and welding in place of the second assemblies in the manner described above, without the leakage of more than a part of the trapped charge of gas from each of the first assemblies. In other words, when each of the heating units 10 in the group mentioned is finished, there is still a slight pressure above atmospheric pressure within the sheath 11 thereof.

At this time, each of the finished heating units 10 is subjected to the usual and conventional testing procedure; which testing procedure in the present instance includes subjecting the finished heating unit to testing in a spectrometer for the purpose of establishing that the two hermetic seals 20 are entirely gas-tight. At this point,

9 'it is mentioned that the spectrometer is exceedingly sensitive to the helium line, whereby the small amount of helium contained in the charge of gas trapped in the sheath 11 may be readily detected in the event of a leakage of one of the seals 20.

In the subsequent testing of the finished heating unit 10, the resistance conductor 12 is energized, with the result that after a short time interval of operation of the heating unit 10, the small amount of air that is trapped in the sheath 11 is depleted. More particularly, the air mentioned is depleted since the contained oxygen oxidizes the interior of the sheath 11 and since the contained nitrogen nitrifies the interior of the sheath 11; and also there is some oxidizing and nitrifying of the resistance conductor 12 and of the terminals 14 and 15. However, the depletion of the small amount of air contained in the sheath 11 is of no consequence, since the inert gases contained therein (helium and argon) are not depleted, so that there is still substantial pressure within the sheath 11, notwithstanding the total depletion of the air initially trapped therein. More particularly, the partial pressure of the inert gases hermetically sealed within the sheath 11 is within the range 150 to 1000 mm. of Hg when the sheath 11 is at an ambient temperature of about 70 F., and this partial pressure of the inert gases hermetically sealed within the sheath 11 is within the range 600 to 4000 mm. of Hg when the sheath 11 is at an operated temperature of about 1650 F.

At this point, it is noted that the fairly wide band of partial pressures mentioned of the inert gases hermetically sealed within the sheath 11 flows from the circumstance that the individual heating units 10 are produced sequentially from the group of first assemblies following the removal thereof from the enclosure; whereby the first of the heating units 10 that is finished contains a higher partial pressure of the inert gases than does the last of the heating uni-ts 10' that is finished. Of course, the first heating unit that is finished contains the higher partial pressure of the inert gases for the simple reason that the leakage time interval of the inert gases through the porous plugs 18 and 19 disposed in the opposite ends of the sheath 11 thereof is minimized.

, The pressure range of the inert gases hermetically sealed in the sheath 11 may extend over a substantial band without materially affecting the thermal conductivity of the mass of granular material 13 so long as the pressure is above about 600 mm. of Hg at the operating temperature of about 1650 F. of the sheath 11 of the finished heating unit 10. On the other hand, should the pressure of the inert gases hermetically sealed in the sheath 11 fall materially below about 600 mm. of Hg at the operating temperature of about 1650 F. of the sheath 11, there is a material reduction in the thermal conductivity of the granular material 13, with the result that the temperature of the resistance conductor 12 is substantially increased with respect to the operating temperature of the sheath 11; which objectionable operating condition causes the resistance conductor 12 to be subject to evaporation and consequent early failure by opencircuit.

In order positively to insure that the pressure of the inert gases that are hermetically sealed in the sheath 11 are well above the critical pressure of about 600 mm. of Hg, as explained above, it is only necessary that the partial pressure of the inert gases hermetically sealed in the sheath 11 fall within the approximate range 150 to 1000 mm. of Hg when the sheath 11 is at an ambient temperature of about 70 R; which objective is readily achieved by utilizing the manufacturing method described above. In other words, after the group of first assemblies have been subjected to the pressure of about 10# per square inch gauge in the enclosure throughout the time interval of about 30 minutes, as previously described, there is a sufiicient content of the inert gases in the sheath 11 of each of the first assemblies so that there is no danger of total leakage thereof from the sheaths 11 during the 10 time interval that is consumed in affixing and welding in place the second assemblies in order to produce the hermetically sealed finished heating units 10, in the manner previously described.

In order to demonstrate the superiority of the hermetically sealed sheathed electric heating units 10 embodying the present invention with respect to conventional sheathed unsealed electric heating units, comparative life tests have been conducted. More particularly, a first group of the hermetically sealed sheathed electric heating units were constructed embodying the present invention and a second group of unsealed sheathed electric heating units were constructed in accordance with the previously mentioned Lien Patent No. 2,962,684; all of the electric heating units being of 1600 watts capacity and being cycled during the life test for 1 hour with the sheath temperature at approximately 1650 F. followed by 20 minutes off. Following 1900 hours of the life test, the insulation resistance of each of the heating units was tested and it was discovered that each of the hermetically sealed heating units maintained an insulation resistance between 0.6 and 0.7 megohm, while each of the unsealed heating units maintained an insulation resistance between 0.04 and 0.05 megohm. Now it is well understood that in an electric heating unit of the sheathed resistance conductor type, it is highly desirable to have a high insulation resistance, because with the high insulation resistance, it is impossible to have a high leakage current between the resistance conductor and the enclosing sheath. On the other hand, with a low insulation resistance, it is possible to have a high leakage current between the resistance conductor and the enclosing sheath. Hence, it is clear that the hermetically sealed heating units described are vastly superior to the unsealed heating units; and in passing, it is mentioned that theseunsealed heating units comprise what may be termed the standard of the electric heating industry.

The electric heating unit 10 of the construction and arrangement described in disclosed and claimed in the previously noted copending application of Eugene F. Dilion.

In view of the foregoing, it is apparent that there has been provided an improved and simplified method of making a sheathed electric heating unit of the hermetically sealed type.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. The method of making a sheathed electric heating unit comprising: providing a first assembly of an elongated tubular metal sheath, an elongated electrical resistance conductor arranged within said sheath and spaced therefrom, an elongated metal terminal arranged at one end of said sheath, the inner end of said terminal beingdisposed interiorly of said one end of said sheath and spaced therefrom and electrically connected to the ad jacent end of said resistance conductor and the outer end of said terminal being disposed exterionly of said one end of said sheath, a compacted mass of granular heatconducting and electrical-insulating material arranged within said sheath and embedding both said resistance conductor and the inner end of said terminal and retaining the same in place in spaced relation with said sheath, and a porous plug of ceramic material arranged in the outer end of said sheath and surrounding the adjacent portion of said terminal and respectively bonded thereto; providing a second assembly of a metal ferrule, a metal tube, the inner end of said tube being disposed interiorly of the outer end of said ferrule and spaced therefrom and the outer end of said tube being disposed exteriorly of the outer end of said ferrule, and a body of gas-impervious and electrical-insulating material arranged within the 1 1 outer end of said ferrule and surrounding the inner end of said tube and providing an hermetic seal therebetween; placing said first assembly in an enclosure; subjecting said first assembly to reduced pressure in said enclosure in order to extract air from said sheath through said porous plug and thus from permeating relation with said compacted mass of granular material; then subjecting said first assembly to a gauge pressure of an inert gas in said enclosure in order to force the inert gas into said sheath through said porous plug and thus into permeating relation with said compacted mass of granular material; then removing said first assembly from said enclosure, whereby the inert gas in said sheath gradually leaks therefrom through said porous plug; placing said second assembly upon said first assembly so that the inner end of said ferrule is disposed in closely surrounding relation with said one end of said sheath and so that the outer end of said ferrule is disposed outwardly of said one end of said sheath and so that said tube is arranged in closely surrounding relation with the outer end of said terminal; producing a first hermetic seal between the inner end of said ferrule and said one end of said sheath; and producing a second hermetic seal between the outer end of said tube and the outer end of said terminal; wherein said second assembly is placed upon said first assembly and said first and second hermetic seals are produced sulficiently promptly following the removal of said first assembly from said enclosure so that a charge of the inert gas is trapped and hermetically sealed in said sheath and into permeating relation with said compacted mass of granular material.

2. The method set forth in claim 1, wherein the inert gas is selected from the class consisting of helium and argon.

3. The method set forth in claim 1, wherein the inert gas consists essentially of argon.

4. The method set forth in claim 1, wherein the inert gas comprises by weight about 96% to 99% argon and 1% to 4% helium.

5. The method set forth in claim 1, wherein the parital pressure of the inert gas hermetically sealed within said sheath is within the range 150 to 1000 mm. of Hg when said sheath is at an ambient temperature of about 70 F.

6. The method set forth in claim 1, wherein the partial pressure of the inert gas hermetically sealed within said sheath is within the range 660 to 4000 mm. of Hg when said sheath is at an operating temperature of about 1650 7. The method set forth in claim 1, wherein said sheath is formed essentially of a nickel-chromium steel, and said ferrule is formed essentially of a low carbon steel.

8. The method set forth in claim 1, wherein said sheath is formed essentially of a nickel-chromium steel comprising by weight about 30% nickel and about chromium and the balance principally iron, and said ferrule is formed essentially of a low carbon steel that is substantially free of sulfur and lead and phosphorus.

9. The method set forth in claim 1, wherein said sheath is formed essentially of chromium-nickel steel, and said ferrule is formed essentially of a low carbon steel.

10. The method set forth in claim 1, wherein said sheath is formed of a chromium-nickel steel comprising by weight about chromium and about 12% nickel and the balance principally iron, and said ferrule is formed essentially of a low carbon steel that is substantially free of sulfur and lead and phosphorus.

11. The method set forth in claim 1, wherein said terminal is formed essentially of a low carbon steel and said tube is formed of a nickel-iron alloy.

12. The method set forth in claim 1, wherein said terminal is formed essentially of a low carbon steel, and said tube is formed of a nickel-iron alloy comprising by weight about 52% nickel and the balance princpally iron.

13. The method set forth in claim 1, wherein said compacted mass of granular material consists essentially of MgO.

14. The method set forth in claim 1, wherein said body of gas-impervious and electrical-insulating material consists essentially of fused glass.

15. The method set forth in claim 14, wherein the thermal coefficient of expansion of said body of fused glass is somewhat lower than that of said ferrule and somewhat higher than that of said tube, whereby said body of fused glass is maintained in compression between said ferrule and said tube.

16. The method set forth in claim 1, wherein said porous plug of ceramic material essentially comprises kaolin and silica and zironium silicate.

17. The method set forth in claim 1, wherein said porous plug of ceramic material accommodates breathing of said inert gas therethrough between said one end of said sheath and the interior of said ferrule.

18. The method set forth in claim 1, wherein said first assembly is heated to an elevated temperature prior to placing the same in said enclosure so that said first assembly is subjected to reduced pressure in said enclosure while it is hot, and wherein said first assembly is allowed to cool substantially while it is in said enclosure and prior to subjecting the same guage pressure of the inert gas in said enclosure.

19. The method set forth in claim 1, wherein said first assembly is subjected to reduced pressure in said enclosure during a time interval of about 30 to 45 minutes by producing a pressure in said enclosure of about 1 mm. Hg, and wherein said first assembly is subjected to a gauge pressure of the inert gas in said enclosure during a time interval of about 30 minutes by producing a pressure in said enclosure of about 10 pounds per sq. in. gauge.

20. The method of making a sheathed electric heating unit comprising: providing a first assembly of an elongated tubular metal sheath, an elongated electricalresistance conductor arranged within said sheath and spaced therefrom, an elongated metal terminal arranged at one end of said sheath, the inner end of said terminal being disposed interiorly of said one end of said sheath and spaced therefrom and electrically connected to the adajacent end of said resistance conductor and the outer end of said terminal being disposed exteriorly of said one end of said sheath, a compacted mass of granular heat-conducting and electrical-insulating material arranged within said sheath and embedding both said resistance conductor and the inner end of said terminal and retaining the same in place in spaced relation with said sheath, and a porous plug of ceramic material arranged in the outer end of said sheath and surrounding the adjacent portion of said terminal and respectively bonded thereto; providing a second assembly of a metal ferrule, a metal tube, the inner end of said tube being disposed interiorly of the outer end of said ferrule and spaced therefrom and the outer end of said tube being disposed exteriorly of the outer end of said ferrule, and a body of gas-impervious and electrical-insulating material arranged within the outer end of said ferrule and surrounding the inner end of said tube and providing an hermetic seal therebetween; placing said first assembly in an enclosure; subjecting said first assembly to reduced pressure in said enclosure in order to extract air from said sheath through said porous plug and thus from permeating relation with said compacted mass of granular material; then subjecting said first assembly to a gauge pressure of an inert gas in said enclosure in order to force the inert gas into said sheath through said porous plug and thus into permeating relation with said compacted mass of granular material; then removing said first assembly from said enclosure, whereby the inert gas in said sheath gradually leaks therefrom through said porous plug; placing said second assembly upon said first assembly so that the inner end of said ferrule is disposed in closely surrounding relation with said one end of said sheath and so that the outer end of said ferrule is disposed outwardly of said one end of said sheath and so that said tube is arranged in closely surrounding relation l3 1 With the -outer end of said terminal; Welding the inner sealed in said sheath and into permeating relation with end of said ferrule and said one end of said sheath in said compacted mass of granular material. order to produce a first hermetic seal therebetween; and Welding the outer end of said tube and the outer end of said terminal in order to produce a second hermetic 5 seal therebetween; wherein said second assembly is placed upon said first assembly and said first and second hermetic seals are produced sufiiciently promptly follow- JOHN F. CAMPBELL, Primary Examineh mg the removal of said first assembly from sa1d enclosure so that a charge of the inert gas is trapped and hermetical- 10 CLINE, Assistant Examiner- References Cited by the Examiner UNITED STATES PATENTS 2,933,805 4/1960 McOrlly 29l55.65 

1. THE METHOD OF MAKING A SHEATHED ELECTRIC HEATING UNIT COMPRISING: PROVIDING A FIRST ASSEMBLY OF AN ELONGATED TUBULAR METAL SHEATH, AN ELONGATED ELECTRICAL RESISTANCE CONDUCTOR ARRANGED WITHIN SAID SHEATH AND SPACED THEREFROM, AN ELONGATED METAL TERMINAL ARRANGED AT ONE END OF SAID SHEATH, THE INNER END OF SAID TERMINAL BEING DISPOSED INTERIORLY OF SAID ONE END OF SAID SHEATH AND SPACED THEREFROM AND ELECTRICALLY CONNECTED TO THE ADJACENT END OF SAID RESISTANCE CONDUCTOR AND THE OUTER END OF SAID TERMINAL BEING DISPOSED EXTERIORLY OF SAID ONE END OF SAID SHEATH, A COMPACTED MASS OF GRANULAR HEATCONDUCTING AND ELECTRICAL-INSULATING MATERIAL ARRANGED WITHIN SAID SHEATH AND EMBEDDING BOTH SAID RESISTANCE CONDUCTOR AND THE INNER END OF SAID TERMINAL AND RETAINING THE SAME IN PLACE IN SPACED RELATION WITH SAID SHEATH, AND A POROUS PLUG OF CERAMIC MATERIAL ARRANGED IN THE OUTER END OF SAID SHEATH AND SURROUNDING THE ADJACENT PORTION OF SAID TERMINAL AND RESPECTIVELY BONDED THERETO; PROVIDING A SECOND ASSEMBLY OF A METAL FERRULE, A METAL TUBE, THE INNER END OF SAID TUBE BEING DISPOSED INTERIORLY OF THE OUTER END OF SAID FERRULE AND SPACED THEREFROM AND THE OUTER END OF SAID TUBE BEING DISPOSED EXTERIORLY OF THE OUTER END OF SAID FERRULE, AND A BODY OF GAS-IMPERVIOUS AND ELECTRICAL-INSULATING MATERIAL ARRANGED WITHIN THE OUTER END OF SAID FERRULE AND SURROUNDING THE INNER END OF SAID TUBE AND PROVIDING AN HERMETIC SEAL THEREBETWEEN; PLACING SAID FIRST ASSEMBLY IN AN ENCLOSURE; SUBJECTING SAID FIRST ASSEMBLY TO REDUCED PRESSURE IN SAID ENCLOSURE IN ORDER TO EXTRACT AIR FROM SAID SHEATH THROUGH SAID POROUS PLUG AND THUS FROM PERMEATING RELATION WITH SAID COMPACTED MASS OF GRANULAR MATERIAL; THEN SUBJECTING SAID FIRST ASSEMBLY TO A GAUGE PRESSURE OF AN INERT GAS IN SAID ENCLOSURE IN ORDER TO FORCE THE INERT GAS INTO SAID SHEATH THROUGH SAID POROUS PLUG AND THUS INTO PERMEATING RELATION WITH SAID COMPACTED MASS OF GRANULAR MATERIAL; THEN REMOVING SAID FIRST ASSEMBLY FROM SAID ENCLOSURE, WHEREBY 